The instant invention relates to a method of fusion welding material, and specifically, relates to a method of fusion welding what are known as metal matrix composites.
A composite may be defined as a man-made material in which two or more constituents are combined to create a material having properties different than those of either constituent. One form of metal matrix composites (MMC) includes those materials which consist of a metal matrix, such as aluminum (Al), cobalt (Co), Iron (Fe) or Magnesium (Mg) or alloys of these metals and a reinforcement material, such as silicon carbide (SiC), tungston carbide (WC), graphite (C) or boron carbide (B.sub.4 C) distributed in the matrix. The reinforcement may be in the form of fibers, wires, whiskers, flakes or particulates.
Composites, such as SiC/Al, B.sub.4 C/Al or B.sub.4 C/Mg are stronger and lighter than their constituents and are particularly abrasion resistant. Secondary processing of metal matrix composites is, however, difficult in that the composites are not easily welded or machined. B.sub.4 C has a hardness, for example, close to that of diamonds, and a composite of B.sub.4 C in a matrix of Mg alloy results in a material which is very difficult to machine.
Conventional fusion welding of MMCs results in extremely porous welds, having little structural strength. (Ahern, et al., Fusion Welding of SiC-Reinforced Al Composites, Metal Construction, Apr. 1982, pp. 192-197.) The matrix or reinforcement materials apparently evolve absorbed gases at normal welding temperatures resulting in gas pockets throughout the weld. One method suggested by Ahern is to bake the composite at temperatures near the matrix melting point to degas the composite.
Rapid solidification methods of welding, such as laser or electron beam welding, produce unsatisfactory results in that the normal matrix and the reinforcement therein are heated to the melting point of the reinforcement, with the results that the metal vaporizes, thereby destroying the composite. The reinforcements generally have melting points of approximately 2500.degree. centigrade while the metal constituents generally have a melting point of 600.degree.-1500.degree. centigrade.
Additionally, if the metal matrix and the reinforcement material reach a high temperature, or if they are maintained for a sufficient length of time at an elevated temperature, which may even be a temperature below the melting point of the matrix, a chemical reaction between the metal and the reinforcement material may occur. In the case of SiC/Al, an aluminum carbide compound may form which destroys the originally intended properties of the composite which is at its strongest when discrete particles, fibers, etc., of SiC are distributed in the aluminum matrix.
Composites may be braze welded but the weld is, of course, no stronger than the filler metal, such as 4043 Al, used in the weld. Filler metal, such as 4043 Al, has less strength than 6061 Al and, because the weld time is fairly long, the previously noted chemical reaction may occur between the matrix and the reinforcement material. Such a procedure may also be of sufficient duration that the matrix or the reinforcement material in the base composite may begin to evolve trapped gases, expanding the base composite, thereby weakening the final structure.
An object of the instant invention is to provide a method of welding metal matrix composite materials which will produce a weld having a strength at least as great as that of the base material.
Another object of the instant invention is to provide a method of welding a metal matrix composite to produce a nonpourous weld.
A further object of the instant invention is to provide a method of welding a metal matrix composite which does not melt the reinforcement constituent thereof.
Another object of the instant invention is to provide a method of welding a metal matrix composite which does not cause a significant chemical reaction between the matrix and the reinforcement material distributed therein.
Still another object of the instant invention is to provide a method of welding a metal to a metal matrix composite.